Plasma treatment of materials is widely used to alter the surface, characteristics of the material. The treatment is generally useful where it is uniformly distributed over the surface of the material. When applied to woof fibres, plasma treatments are used to oxidize a lipid layer on the surface of the fibres. Oxidation of the lipid layer makes the wool fibres more receptive to subsequent surface treatments, for instance anti-shrink and pilling-prevention treatments.
Removal of the lipid layer also increases friction between the fibres. This benefits yarn production processes as less twisting is required to form the yarn. The lower twisting level enables the yarn to be produced at a greater rate, enabling downstream processes to be run faster and thereby increasing output. Additionally, yarns having a lower twist exhibit a softer feel relative to higher twist yarns and may be used advantageously to produce commercially desirable softer garments in contrast to garments made from high twist yarn.
Plasma treatment of wool and other fibrous materials should provide an even surface treatment to ensure that the material is receptive to downstream processing in a production line. If the surface of the material is not treated evenly, the downstream processes will not have their designed effect and an inferior product will result.
Another aspect of plasma treatment is that wool and other fibrous materials are susceptible to being locally burnt during the plasma treatment. Again, in a production line, this is highly undesirable where a continuous supply of material is required. Hence, an optimum plasma treatment should minimise the incidence of localised burning of the material being treated.
Some of the present techniques for the generation of plasma for treating materials involves the adjustment of the applied voltage and its frequency in order to obtain a stable uniform plasma. Such plasma is usually generated with gas pressures above or below atmospheric pressure. More recently, advances in plasma treatments at atmospheric pressure have involved the use of expensive noble gases to stabilise the plasma in a uniform glow suitable for surface treating materials. The cost of plasma treatments above or below atmospheric pressure or involving noble gases makes them less economically viable. Accordingly, interest has been focussed on plasma treatments in air and at atmospheric pressure.
Roth et al, in U.S. Pat. No. 5,403,453, teaches that a uniform glow discharge plasma is created at one atmosphere where ions, produced by the electrical breakdown of helium and/or air, are trapped between the electrodes. Roth indicates that the ion trapping increases the lifetime of the ions in the plasma and thereby results in a lower electrical breakdown threshold and a uniform glow discharge. A similar theory is posited in U.S. Pat. No. 6,299,948 to Gherardi et al assigned to L'Air Liquide. According to Roth et al., such trapping is enabled by applying an electrical field alternating at radio frequencies between spaced electrodes. Roth et al. propose a relationship between electrode spacing, electrode voltage and applied frequency that results in ion trapping.
International patent publication WO 02/094455 discloses a plasma treatment at atmospheric pressure in which a spatially homogenous distribution of microdischarges is achieved over an elongated discharge electrode, by introducing a gas stream at an inclination to the longitudinal axis of symmetry. The relatively complex structure involves multiple channels from a gas distribution chamber.
U.S. Pat. No. 5,895,558 describes a plasma treatment station for polymer strip at about atmospheric pressure, but preferably slightly superatmospheric pressure, in which gas is blown from an array of holes in one of a pair of closed-spaced planar electrodes. Turbulent high flow gas delivery is employed to delay and disrupt the formation of a filamentary discharge.
It is an object of the invention to provide an improved plasma treatment technique for use with gas permeable materials.